Dr. G. J. Stoney on Microscopic Vision. 507 



thing that is determined by the positions of the transversals 

 of these beams, or the phases of the luminous waves. 



35. Resolution. — Our first illustration of the great assistance 

 which is rendered to the practical microscopist by Abbe's 

 theory will be taken from the guidance it gives him when he 

 wants to set up his apparatus so as to resolve an unusually 

 difficult " test-object." That which we shall take is Amphi- 

 pleura pellucida. This cigar-shaped diatom is less than the 

 tenth of a millimetre in length, and about a tenth of that 

 again in width. With ordinary objectives the only detail 

 seen on it is the ridge surrounding it and a longitudinal 

 midrib which dilates into loops at the two ends. Between 

 the midrib and the sides of the diatom are what seem to be 

 two mere uniform plains ; but when adequate arrangements 

 are made each of these plains proves to be covered by trans- 

 verse rows of specks so close that Mr. Nelson in one specimen 

 counted 96 of these rows in the thousandth of an English 

 inch ; and the specks of which the rows consist are somewhat 

 closer still. Hence cr, the spacing of the rows, is the 96,000th 

 part of an English inch, which is the same as <r = 0*265 fju, 

 where fi stands for the micron, the thousandth part of a 

 millimetre. This is a good deal shorter than the shortest 

 wave-length of the most extreme ultra-violet light which can 

 reach us from the sun. The wave-lengths of the visible part 

 of light are much larger, ranging from 0*38 /jl to 0*76 [x. 



The diatom was one mounted by Professor Van Heurck 

 in a medium containing arsenic disulphide (As 2 S 2 ), the 

 refractive index of which is said to be 2*4 ; so that the 

 relative refractive index between it and silex is about 1*7, 

 thus affording both the advantage described in Theorem 6, 

 p. 348, and what is in the present case the still greater 

 advantage described on the same page, in the paragraph 

 which follows the enunciation of that theorem. Moreover, 

 the result seems to show that this medium has also a relatively 

 low dispersive power, which adds to its value. It is unfor- 

 tunate that a material possessed of these important properties 

 is so difficult to manipulate, or so risky, that no one seems 

 to have succeeded in mounting objects in it except Dr. Van 

 Heurck himself. 



The apparatus employed were an immersion objective 

 of which the nominal G (numerical aperture or grasp) was 

 1-35, and a dry condenser of which G^O'9. To be on the 

 safe side it was thought well to reckon only on being able 

 to make 1*25 of the grasp of the objective available, and 0*85 

 of the grasp of the condenser. Putting these values for 

 g and g' into equation (2), viz. : — 



\=v(ff+ff'), (2) 



